Bistable nematic liquid crystal device

ABSTRACT

A bistable nematic liquid crystal device cell is provided with a surface alignment grating on at least one cell wall and a surface treatment on the other wall. Such treatment may be a homeotropic alignment or a planar alignment with or without an alignment direction, and zero or a non zero pretilt. The surface profile on the monograting is asymmetric with its groove height to width selected to give approximately equal energy within the nematic material in its two allowed alignment arrangements. The monograting may be formed by a photolithographic process or by embossing of a plastics material. The cell is switched by dc pulses coupling to a flexoelectric coefficient in the material, or by use of a two frequency addressing scheme and a suitable two frequency material. Polarisers either side of the cell distinguish between the two switched states. The cell walls may be rigid or flexible, and are coated with electrode structures, e.g. in row and column format giving an x,y matrix of addressable pixels on the cell.

This invention relates to bistable nematic liquid crystal devices.

Liquid crystal devices typically comprise a thin layer of a liquidcrystal material contained between cell walls. Optically transparentelectrode structures on the walls allow an electric field to be appliedacross the layer causing a re-ordering of the liquid crystal molecules.

There are three known types of liquid crystal material, nematic,cholesteric, and smectic each having a different molecular ordering. Thepresent invention concerns devices using nematic materials.

In order to provide displays with a large number of addressable elementsit is common to make the electrodes as a series of row electrode on onewall and a series of column electrodes on the other cell wall. Theseform e.g. an x, y matrix of addressable elements or pixels and, fortwisted nematic types of devices, are commonly addressed using rms.addressing methods.

Twisted nematic and phase change type of liquid crystal devices areswitched to an ON state by application of a suitable voltage, andallowed to switch to an OFF state when the applied voltage falls below alower voltage level, i.e. these devices are monostable. For a twistednematic type of device (90° or 270° degree twist as in U.S. Pat. No.4,596,446), the number of elements that can be rms. addressed is limitedby the steepness of a device transmission vs voltage curve as details byAlt and Pleschko in IEEE Trans ED vol ED 21 1974 pages 146-155. One wayof improving the number of pixels is to incorporate thin filmtransistors adjacent each pixel; such displays are termed active matrixdisplays. An advantage of nematic type of devices is the relatively lowvoltage requirements. They are also mechanically stable and have widetemperature operating ranges. This allows construction of small andportable battery powered displays.

Another way of addressing large displays is to use a bistable liquidcrystal device. Ferroelectric liquid crystal displays can be made intobistable device with the use of smectic liquid crystal materials andsuitable cell wall surface alignment treatment. Such a device is asurface stabilised ferroelectric liquid crystal device (SSFELCDs) asdescribed by:—L J Yu, H Lee, C S Bak and M M Labes, Phys Rev Lett 36, 7,388 (1976); R B Meyer, Mol Cryst Liq Cryst. 40, 33 (1977); N A Clark andS T Lagerwall, Appl Phys Lett, 36, 11, 899 (1980). One disadvantage offerroelectric devices is the relatively large voltage needed to switchthe material. This high voltage makes small portable, battery powereddisplays expensive. Also these displays suffer from other problems suchas lack of shock resistance, limited temperature range and alsoelectrically induced defects such as needles.

If bistable surface anchoring can be achieved using nematics then adisplay can be made which has the merits of both the above mentionedtechnologies but none of the problems.

It has already been shown by Durand et al that a nematic can be switchedbetween two alignment states via the use of chiral ions or flexoelectriccoupling: A Charbi, R Barberi, G Durand and P Martinot-Largarde, PatentApplication No WO 91/11747, (1991) “Bistable electrochirally controlledliquid crystal optical device”, G Durand, R Barberi, M Giocondo, PMartinot-Largarde, Patent Application No WO 92/00546 (1991) “Nematicliquid crystal display with surface bistability controlled by aflexoelectric effect”. These are summarised as follows:

In Patent Application No WO 91/11747 a device is described with thefollowing characteristics:

-   1. The cell is made using two surfaces which have SiO coatings of    appropriate thickness and evaporation angle to allow two stable    states to exist on each surface. Furthermore the two states on a    surface are designed to differ in azimuthal angle by 45° and the    surfaces are oriented to differ in azimuthal angle by 45° and the    surfaces are oriented such that each of the two resulting domains    are untwisted.-   2. The cell (of 6 μm thickness) is filled with 5CB doped with 0.5%    benzyl quininium bromide and 1.8% phenyl lactic acid. The former is    an electrically positive chiral ion with left hand twist while the    latter is a negative chiral ion with a right hand twist. The    concentrations ensure that the final mixture has a very long pitch    so that the states in the thin cell are uniform.-   3. Application of a 110V dc pulse for 40 μs enabled switching    between the two states. A lower threshold is observed for longer    pulse e.g. an 80V threshold is observed for 300 μs pulses.-   4. Addition of suitably oriented polarisers caused one state to    appear black while the other appears white with a contrast ratio of    about 20.-   5. A variant device is also mentioned which causes a short pitch    chiral ion mixture between monostable surfaces which possess    different zenithal anchoring energies. Switching between a 180°    twisted state and a uniform state is observed in a 4 μm cell for    pulses over 50V.

In Patent Application WO 92/00546 a device is described with thefollowing characteristics:

The cell is made using two surfaces which have SiO coatings ofappropriate thickness and evaporation angle to allow two stable statesto exist on each surface. Furthermore the two states on a surface aredesigned to differ in azimuthal angle by 45° and the surfaces areoriented such that each of the two resulting domains are untwisted.

The surfaces are also oriented in such a way that the pretilted state onone surface lines up with the untilted state on the other surface andvice versa. Hence when filled with 5CB, the two states are seen as shownin FIGS. 7B and 7C.

Application of a 14V dc pulse across a 1 μm cell for 100 μs allowsswitching between the states. The final state is dependent on the signof the pulse due to its coupling to the flexoelectric polarisation. Thesame voltage threshold is observed for switching in both directions.

The surface used by Durand to obtain bistable alignment was a thin layerof SiO evaporated at a precise oblique angle. However this methodsuffers the disadvantage that any deviation in the evaporation angle,layer thickness or indeed any of the deposition parameters is likely toproduce a surface with only monostable alignment. This makes the obliqueevaporation technique unsuitable, or very difficult, for large areadisplays.

U.S. Pat. No. 4,333,708 describes a multistabl liquid crystal device inwhich cell walls are profiled to provide an array of singular points.Such substrate configurations provide multistable configurations of thedirector alignments because disclination must be moved to switch betweenstable configurations. Switching is achieved by application of electricfields.

Another bistable nematic device is described in GB.2,286,467-A. Thisuses accurately formed bigratings on at least one cell wall. Thebigrating permits liquid crystal molecules to adopt two differentangular aligned directions when suitable electrical signals are appliedto cell electrodes, e.g. dc coupling to flexoelectric polarisation asdescribed in Patent Application No. WO.92/00546. Since in the twosplayed state the director is quite close to being in the plane of thelayer, the coupling between director and flexoelectric component can besmall, which may hinder switching in some circumstances.

According to this invention the above disadvantages are overcome by asurface treatment to at least one cell wall that permits nematic liquidcrystal molecules to adopt either of two pretilt angles in the sameazimuthal plane. The cell can be electrically switched between these twostates to allow information display which can persist after the removalof power.

The term same azimuthal plane is explained as follows; let the walls ofa cell lie in the x,y plane, which means the normal to the cell walls isthe z axis. Two pretilt angles in the same azimuthal plane means twodifferent molecular positions in the same x,z plane

According to this invention a bistable nematic liquid crystal devicecomprises;

-   two cell walls enclosing a layer of liquid crystal material;-   electrode structures on both walls;-   a surface alignment on the facing surfaces of both cell walls    providing alignment to liquid crystal molecules;-   means for distinguishing between switched states of the liquid    crystal material:    CHARACTERISED BY-   a surface alignment grating on at least one cell wall that permits    the liquid crystal molecules to adopt two different pretilt angles    in the same azimuthal plane;-   the arrangement being such that two stable liquid crystal molecular    configurations can exist after suitable electrical signals have been    applied to the electrodes.

The grating may have a symmetric or an asymmetric groove profile.

The grating may have an asymmetric groove profile which will induce apretilt of less than 90°, e.g. 50° to 90°. An asymmetric profile may bedefined as a surface for which there does not exist a value of h suchthat;Ψ_(x)(h−x)=Ψ_(x)(h+x)  (1)for all values of x, where Ψ is the function describing the surface.

The gratings may be applied to both cell walls and may be the same ordifferent shape on each wall. Furthermore the grating profile may varywithin each pixel area, and or in the inter pixel gaps betweenelectrodes. One or both cell walls may be coated With a surfactant suchas lethecin.

The liquid crystal material may be non twisted in one or both stablemolecular configurations.

The cell walls may be formed of a relatively thick non flexible materialsuch as a glass, or one or both cells walls may be formed of a flexiblematerial such as a thin layer of glass or a plastic material flexiblee.g. polyolefin or polypropylene. A plastic cell wall may be embossed onits inner surface to provide a grating. Additionally, the embossing mayprovide small pillars (e.g. of 1-3 μm height and 5-50 μm or more width)for assisting in correct spacing apart of the cell walls and also for abarrier to liquid crystal material flow when the cell is flexed.Alternatively the pillars may be formed by the material of the alignmentlayers.

The grating may be a profiled layer of a photopolymer formed by aphotolithographic process e.g. M C Hutley, Diffraction Gratings(Academic Press, London 1982) p 95-125; and F Horn, Physics World, 33(March 1993). Alternatively, the bigrating may be formed by embossing; MT Gale, J Kane and K Knop, J App. Photo Eng, 4, 2, 41 (1978), or ruling;E G Loewen and R S Wiley, Proc SPIE, 88 (1987), or by transfer from acarrier layer.

The electrodes may be formed as a series of row and column electrodesarranged and an x,y matrix of addressable elements or display pixels.Typically the electrodes are 200 μm wide spaced 20 μm apart.

Alternatively, the electrodes may be arranged in other display formatse.g. r-θ matrix or 7 or 8 bar displays.

The invention will now be described, by way of example only withreference to the accompanying drawings of which;

FIG. 1 is a plan view of a matrix multiplexed addressed liquid crystaldisplay;

FIG. 2 is the cross section of the display of FIG. 1;

FIG. 3 shows a top view and a side view of the mask and exposuregeometry used to produce a grating surface.

FIG. 4 is a cross section of the liquid crystal director configurationon the grating surface which leads to a higher pretilt.

FIG. 5 is a cross section of the liquid crystal director configurationon the grating surface which leads to a lower pretilt.

FIG. 6 is the energy of the two pretilt configurations as a function ofgroove depth to pitch ratio (h/w).

FIG. 7 shows a cross section of a cell configuration which allowsbistable switching between the two states.

FIG. 8 shows the transmission of the cell and the applied signals as afunction of time.

FIG. 9 shows an example multiplexing scheme for the bistable device.

FIG. 10 shows an alternative cell configuration for bistable switching.

FIG. 11 shows a cell configuration for bistable switching between anon-twisted and a twisted state.

The display in FIGS. 1, 2 comprises a liquid crystal cell 1 formed by alayer 2 of nematic or long pitch cholesteric liquid crystal materialcontained between glass walls 3, 4. A spacer ring 5 maintains the wallstypically 1-6 μm apart. Additionally numerous beads of the samedimensions may be dispersed within the liquid crystal to maintain anaccurate wall spacing. Strip like row electrodes 6 e.g. of SnO₂ or ITO(indium tin oxide) are formed on one wall 3 and similar columnelectrodes 7 are formed on the other wall 4. With m-row and n-columnelectrodes this forms an m×n matrix of addressable elements or pixels.Each pixel is formed by the intersection of a row and column electrode.

A row driver 8 supplies voltage to each row electrode 6. Similarly acolumn driver 9 supplies voltages to each column electrode 7. Control ofapplied voltages is from a control logic 10 which receives power from avoltage source 11 and timing from a clock 12.

Either side of the cell 1 are polarisers 13, 13′ arranged with theirpolarisation axis substantially crossed with respect to one another andat an angle of substantially 45° to the alignment directions R, if any,on the adjacent wall 3, 4 as described later. Additionally an opticalcompensation layer 17 of e.g. stretched polymer may be added adjacent tothe liquid crystal layer 2 between cell wall and polariser.

A partly reflecting mirror 16 may be arranged behind the cell 1 togetherwith a light source 15. These allow the display to be seen in reflectionand lit from behind in dull ambient lighting. For a transmission device,the mirror 16 may be omitted.

Prior to assembly, at least one of the cell walls 3, 4 are treated withalignment gratings to provide a bistable pretilt. The other surface maybe treated with either a planar (i.e. zero or a few degrees of pretiltwith an alignment direction) or homeotropic monostable surface, or adegenerate planar surface (i.e. a zero or few degrees of pretilt with noalignment direction).

Finally the cell is filled with a nematic material which may be e.g. E7,ZLI2293 or TX2A (Merck).

An example method used to fabricate the grating surface will nowdescribed with reference to FIG. 3.

EXAMPLE 1

A piece of ITO coated glass to form the cell wall 3, 4 was cleaned withacetone and isopropanol and was then spin coated with photoresist(Shipley 1805) at 3000 rpm for 30 seconds giving a coating thickness of0.55 μm. Softbaking was then carried out at 90° C. for 30 minutes.

A contact exposure was then carried out on the coated wall 3, 4 using achrome mask 20 containing 0.5 μm lines 21 and 0.5 μm gaps 22 (hence anoverall pitch of 1 μm) as shown in FIG. 3. The exposure was carried outat non-normal incidence, in this case an angle of 60° was used. Mask 20orientation is such that the groove direction is substantiallyperpendicular to the to plane of incidence as shown in FIG. 3. Exposurein this geometry leads to an asymmetric intensity distribution andtherefore an asymmetric grating profile (see for example B. J. Lin, J.Opt. Soc. Am., 62, 976 (1972)). Coated cell walls 3, 4 were exposed tolight from a mercury lamp (Osram Hg/100) with an intensity of 0.8 mW/cm²for a period of about 40 to 180 seconds as detailed later.

After the exposure the coated cell wall 3, 4 was released from the mask20 and developed in Shipley MF319 for 10 seconds followed by a rinse inde-ionised water. This left the cell wall's surface patterned with anasymmetric surface modulation forming the desired grating profile. Thephotoresist was then hardened by exposure to deep UV radiation (254 nm)followed by baking at 160° C. for 45 minutes. This was done to ensureinsolubility of the photoresist in the liquid crystal. Finally thegrating surface is treated with a solution of the surfactant lecithin inorder to induce a homeotropic boundary condition.

Finite element analysis has been carried out in order to predict themolecular (more correctly the director) configuration of a free layer ofnematic material on such grating surfaces. The results are shown inFIGS. 4, 5 and 6 where the short lines represent liquid crystal directorthroughout the layer thickness, with the envelope of the short lines atthe bottom showing the grating profile. In this case the grating surfacehas been described by the function; $\begin{matrix}{{y(x)} = {\frac{h}{2}{\sin\left( {\frac{2\pi\quad x}{w} + {A\quad{\sin\left( \frac{2\pi\quad x}{w} \right)}}} \right)}}} & (2)\end{matrix}$where h is the groove depth, w is the pitch and A is an asymmetryfactor. In FIGS. 4 and 5, A=0.5 and h/w=0.6. In FIG. 4, the finiteelement grid has been allowed to relax from an initial director tilt of80°. In this case the configuration has relaxed to a pretilt of 89.5°.However, if the initial director tilt is set to 30° then the gridrelaxes to a pretilt of 23.0° as shown in FIG. 5. Therefore the nematicliquid crystal can adopt two different configurations depending onstarting conditions.

In practice a nematic liquid crystal material will relax to whichever ofthese two configuration has the lowest overall distortion energy. FIG. 6shows the total energy (arbitrary units) of the high pretilt (solidcircles) and the low pretilt (empty circles) state verses the groovedepth to pitch ratio (h/w). For low h/w, the high pretilt state has thelowest energy and so the nematic will adopt a high pretilt state.Conversely for large h/w, the low pretilt state has the lowest energyand so this state is formed. However when h/w=0.52, the states have thesame energy and so either can exist without relaxing into the other.Therefore if a surface is fabricated at, or close to this condition,then bistability can be observed in the pretilt. With reference to theabove fabrication details, an exposure time of 80 seconds was found tolead to a bistable surface. In this case the bistability is purely afunction of the surface and does not rely on any particular cellgeometry. In this sense it is distinct from prior art such as U.S. Pat.No. 4,333,708 (1982).

One suitable cell configuration to allow switching between the bistablestates is shown in FIG. 7 which is a stylised cross section of thedevice in which a layer 2 of nematic liquid crystal material withpositive dielectric anisotropy is contained between a bistable gratingsurface 25 and a monostable homeotropic surface 26. The latter surface26 could, for example, be a flat photoresist surface coated withlecithin. Within this device liquid crystal molecules can exist in twostable states. In state (a) both surfaces 25, 26 are homeotropic whereasin (b) the grating surface 25 is in its low pretilt state leading to asplayed structure. For many nematic materials, a splay or benddeformation will lead to a macroscopic flexoelectric polarization whichis represented by the vector P in FIG. 7. A dc pulse can couple to thispolarisation and depending on its sign will either favour or disfavourconfiguration (b).

With the device in state (a), the application of a positive pulse willstill cause fluctuations in the homeotropic structure despite thepositive dielectric anisotropy. These fluctuations are sufficient todrive the system over the energy barrier that separates the twoalignment states. At the end of the pulse the system will fall intostate (b) because the sign of the field couples favourably with theflexoelectric polarisation. With the system in state (b), a pulse of thenegative sign will once again disrupt the system but now it will relaxinto state (a) as its sign does not favour the formation of theflexoelectric polarisation. In its homeotropic state, the bistablesurface is tilted at slightly less than 90° (e.g. 89.5°). This issufficient to control the direction of splay obtained when the cellswitches into state (b).

One particular cell consisted of a layer of nematic ZLI2293 (Merck)sandwiched between a bistable grating surface and a homeotropic flatsurface. The cell thickness was 3 μm. Transmission was measured throughthe cell during the application of dc pulses at room temperature (20°C.). The polariser and analyser 13, 13′ on each side of the cell 1 werecrossed with respect to each other and oriented at ±45° to the gratinggrooves. In this set up, the two states in FIG. 7, (a) and (b), appearblack and white respectively when addressed as follows.

FIG. 8 shows the applied voltage pulses (lower trace) and the opticalresponse (upper trace) as a function of time. Each pulse had a peakheight of 55.0 volts and a duration of 3.3 ms. Pulse separation was 300ms. With the first application of a positive pulse, the transmissionchanges from dark to light indicating that the cell has switched fromstate FIG. 7(a) to state (b). A second positive pulse causes a transientchange in transmission due to the rms effect of coupling to the positivedielectric anisotropy causing a momentary switching of the bulk materialto state (a). However, in this case the cell does not latch at thesurface and so remains in state (b). The next pulse is negative in signand so switches the cell from state (b) to state (a). Finally a secondnegative pulse leaves the cell in state (a). This experiment shows thatthe cell does not change state on each pulse unless it is of the correctsign. Thus it proves that the system is bistable and that the finalstate can be reliably selected by the sign of the applied pulse.

The switching occurs across a wide temperature range. As the temperatureis increased the voltage required for switching falls. For example at30° C., a voltage of 44.8 V is required for bistable switching whereasat 50° C. the voltage in only 28.8 V. Similarly, for a fixed voltage therequired pulse length for latching decreases with temperature.

After this data was taken, the cell was dismantled and the gratingsurface as characterised by AFM (atomic force microscopy). An asymmetricmodulation was confirmed which was fitted by equation 2 to give a pitchof 1 μm, a groove depth of 0.425 μm (h/w=0.425) and an asymmetry factorof A=0.5. In comparison to the results in FIG. 6, this grating has itsbistable regime at a lower value of h/w (0.425 compared to 0.52).However equation 2 was not a precise fit to the AFM data due to the realsurface possessing steeper facet angles which require the addition ofhigher harmonics in the description. Other effects such as AFM tipradius also need to be considered for a more accurate comparison. Thusit can be concluded that the measured surface modulation is similar tothe predicted regime for bistability.

The successful switching of a single pixel allows the design of asuitable multiplexing method for the selection of several adjacentpixels. FIG. 9 shows one particular example of such a scheme. As shownpixels in four consecutive rows R1, R2, R3, R4 in one column are to beswitched. Two possible alignment states may be arbitrarily defined as ONand OFF states. Rows R1 and R4 are to switched to an ON state, rows R2and R3 are in the OFF state. Strobe pulses of +V_(s) for three timeslots followed by −V_(s) for 3 time slots (ts) are applied to each rowin turn. A data waveform is applied to the column as shown and comprisesa −V_(d) for 1 ts followed by a +V_(d) for 1 ts for and ON pixel, and−V_(d) for 1 ts followed by +V_(d) for 1 ts for and OFF pixel.

Now considering one particular pixel at A. The resultant waveformconsists of large positive and negative pulses which disrupt the nematicorientation and raises its energy up to the barrier that separates thetwo bistable surface states. In this field applied conditon, the liquidcrystal molecules align along the electric field as in conventionalmonostable nematic devices, and as shown in FIG. 7 a. These large‘reset’ pulses of opposite polarity are followed immediately by asmaller pulse which is still large enough to dictate the final selectionstate of the pixel during the relaxation of the orientation. Electricalbalance is achieved by a small pulse of polarity opposite to theswitching pulse and preceding the two large pulses. Alternatively,polarity inversion in adjacent display address time may be used.

The above bistable device achieves final state selection by virtue ofthe flexoelectric polarisation in one state. Therefore thisconfiguration must contain splay. In the experimental example only onesurface is allowed to switch but working devices can also be made inwhich both surfaces switch. The only remaining constraint is that thelow pretilt states on each surface should differ in value so that afinite splay remains. However even if the low pretilt states are equal,the cell can still be switched if it contains a two frequency nematicmaterial, that is a material whose dielectric anisotropy is positive atlow frequencies and negative at high frequencies. An example of such amaterial is TX2A (Merck) which has a crossover frequency of 6 kHz. FIG.9 shows a cross section of this configuration. With the cell in state(a), the application of a high frequency signal drives the bulk of thenematic to a low pretilt. The surfaces follow and so the cell switchesto state (b). Conversely a low frequency signal will drive the nematicto a high pretilt and so the cell will switch to state (a).

EXAMPLE 2

A second example of a bistable device is now described. A piece of ITOcoated glass to form the cell wall was cleaned with acetone andisopropanol and was then spin coated with photoresist (Shipley 1813) at3000 rpm for 30 seconds giving a coating thickness of 1.5 μm. Softbakingwas then carried out at 90° C. for 30 minutes.

A contact exposure was then carried out using a chrome mask containing0.5 μm lines and 0.5 μm gaps (hence an overall pitch of 1 μm). In thisexample the exposure was carried out at normal incidence. Exposure inthis geometry leads to a symmetric intensity distribution and thereforea symmetric grating profile. Samples were exposed to light from amercury amp (Osram Hg/100) with an intensity of 0.8 mW/cm².

After the exposure the sample was released from the mask and developedin Shipley MF319 for 20 seconds followed by a rinse in de-ionised water.This left the sample patterned with a symmetric surface modulation. Thephotoresist was then hardened by exposure to deep UV radiation (254 nm)followed by baking at 160° C. for 45 minutes. This was done to ensureinsolubility of the photoresist in the liquid crystal. Finally thegrating surface is treated with a solution of a chrome complexsurfactant in order to induce a homeotropic boundary condition.

One particular surface was made using the above method with an exposuretime of 360 s. AFM analysis on this grating showed it to have asymmetric profile with a pitch of 1 μm and a depth of 1.2 μm. Thissurface was constructed opposite a flat homeotropic surface to form acall with a thickness of 2.0 μm. The cell was filled with the nematicmaterial E7 (Merck) in the isotropic phase followed by cooling to roomtemperature. Microscopic observation revealed a mixture of both bistablestates, shown as (a) and (b) in FIG. 7.

The cell was oriented between crossed polarisers so that the groovedirection was at 45° to the polariser directions. Thus state (a) was thebright state while state (b) was the dark state. Monopolar pulses ofalternating sign were then applied to the cell. The pulse length was setto 5.4 ms with a 1 s pulse separation. Full switching occurred betweenstate (a) and (b) when the peak voltage of the applied pulses wasincreased to 20.3 V. Pairs of pulses were also applied to the cell in asimilar manner to the data shown in FIG. 8. Once again only the firstpulse changed the state of the system while the second pulse onlyinduced a non-latching transient response. In this case the opticalresponse times were also measured. The 10%-90% response time forswitching from (a) to (b) was 8.0 ms while the response time forswitching from (b) to (a) was 1.2 ms. Further analysis of this cellrevealed that the bistable states (a) and (b) induced pretilts of 90°and 0° respectively on the grating surface. Thus this sample hasdemonstrated the maximum possible change in pretilt.

The optics of the configurations shown in FIGS. 7 and 10 is optimisedwhen the cell thickness d is given by:— $\begin{matrix}{d = \frac{\lambda}{2\Delta\quad n_{av}}} & (3)\end{matrix}$where λ is the operating wavelength and Δn_(av) is the average value ofthe in-plane component (parallel to the cell walls) of the nematicbirefringence. Δn_(av) will be larger for the configuration shown inFIG. 10 compared to FIG. 7, hence the cell thickness can be less andtherefore the optical switching speed will be larger. However the use ofa two frequency nematic limits the choice of available materials, alsoleads to a more complex addressing scheme, but may allow lower voltageoperation.

EXAMPLE 3

The bistable grating surface can also be constructed opposite a planarsurface. One such cell consisted of a grating with the same profile tothat described in example 2. This was constructed opposite a rubbedpolymer surface formed using a layer of PI32 polyimide (Ciba Geigy). Therubbing direction on the polyimide surface was set parallel to thegrating groove direction on the grating surface. The cell gap was set to2.5 μm and nematic E7 was used to fill the cell. Cooling to roomtemperature after filling revealed two states which are shownschematically in FIG. 11. This Figure differs from FIG. 7 in that thegroove direction on the bistable surface is now in the plane of the page(in an x,y plane). Thus the 90° pretilt state on the grating forms thehybrid structure shown in (a′) while the 0° pretilt state on the gratingforms the twisted structure shown in (b′). To achieve optical contrastbetween the states, the cell was placed in-between crossed polarisers13, 13′ oriented so that the grating grooves (and rubbing direction)were parallel to one polariser, although the polarisers may be rotatedto optimise contrast in the two switch states. Thus state (b′) was thebright state while state (a′) was the dark state. Using 5.3 ms monopolarpulses, switching between (a′) and (b′) occurred at a peak voltage of56.7 V. The optical response times were 110 ms for switching from (a′)to (b′) and 1.4 ms for switching from (b) to (a′).

The bright state (b′) has a bulk twist of 90°. As with conventional TNstructures, the maximum transmission is obtained when N is an integerwhere (C. H. Gooch and H. A. Tarry, J. Phys. D: Appl. Phys., 8 1575(1975));N={square root}{square root over ((Δnd/λ)²+0.25)}  (4)where Δn is the nematic birefringence, d is the cell gap and λ is theoperating wavelength. Therefore a bistable device using E7 (Δn=0.22)with an operating wavelength of 530 nm and N=1 will have a cell gap of2.1 μm.

In comparison the configuration described in example 2 has an optimumthickness given by equation 3. For that example, Δn_(av) is Δn/2therefore equation 3 gives a thickness of 1.2 μm. Thus the bistabledevice without twist will always possess optimum optics at a thinnercell gap and will therefore switch at lower voltages with a shorteroptical response time.

A cholesteric dopant (eg <1% of CB15 Merck) may be added to preventtwist disclinations. Alternatively these disclinations may be preventedby arranging the groove directions non parallel to the rubbing alignmentdirections, eg about 5° adjustment.

The grating surfaces for these devices can be fabricated using a varietyof techniques as listed earlier. The homeotropic treatment can be anysurfactant which has good adhesion to the grating surface. Thistreatment should also lead to an unpinned alignment. That is, analignment which favours a particular nematic orientation withoutinducing rigid positional ordering of the nematic on the surface.

As seen from the above analysis, the grating modulation has to possess acertain h/w for a given asymmetry for bistability to exist. The absolutescale of the modulation is limited by other factors. If the groove depthand pitch are too large then diffractive effects will be significant andlead to loss of device throughput. Furthermore if the groove depth issimilar to the cell thickness then the proximity of the groove peaks tothe opposite flat surface may inhibit bistable switching. If twogratings are required as in the device shown in FIG. 10 then a largegroove depth compared to cell thickness would inevitably lead toswitching which depends on the phase of the two modulations. This wouldseverely complicate the device manufacturing process.

Problems also exist if the groove depth and pitch are too small. For aconstant h/w, as the pitch becomes smaller the energy density of thebulk distortion at the surface becomes larger. Eventually this energy issimilar to the local anchoring energy of the nematic on the surface.Thus the structures shown in FIGS. 4 and 5 (which assume an infiniteanchoring energy) would no longer be obtained and bistability wouldinevitably be lost. Typical values of h and w are, about 0.5 μm and 1.0μm in a range of about 0.1 to 10 μm and 0.05 to 5 μm respectively.

Small amounts e.g. 1-5% of a dichroic dye may be incorporated into theliquid crystal material This may be used with or without a polariser, toprovide colour, to improve contrast, or to operate as a guest host typedevice; e.g. the material D124 in E63 (Merck). The polariser(s) of thedevice (with or without a dye) may be rotated to optimise contrastbetween the two switched states of the device.

1-16. (Cancelled)
 17. A bistable nematic liquid crystal devicecomprising; a first cell wall and a second cell wall, said first cellwall and said second cell wall enclosing a layer of liquid crystalmaterial, wherein said first cell wall has a first surface treated toprovide a bistable pretilt to molecules of liquid crystal material andsaid second cell wall has a first surface treated to provide monostablealignment to molecules of liquid crystal material, wherein said bistablenematic liquid crystal device provides two stable and opticallydistinguishable liquid crystal configurations.
 18. A device according toclaim 17 wherein the first surface of the second cell wall is treatedwith one of a planar, a degenerate planar or a homeotropic surfacetreatment.
 19. A device according to claim 17 wherein said layer ofliquid crystal material comprises a nematic liquid crystal material 20.A device according to claim 17 wherein said layer of liquid crystalmaterial comprises a long pitch cholesteric liquid crystal material. 21.A device according to claim 17 wherein the first surface of the firstcell wall comprises a plurality of pillars.
 22. A device according toclaim 21 wherein the height of each of said plurality of pillars iswithin the range of 1-3 μm.
 23. A device according to claim 21 whereinthe width of each of said plurality of pillars is within the range of5-50 μm.
 24. A device according to claim 21 wherein the width of each ofsaid plurality of pillars is greater than 50 μm.
 25. A device accordingto claim 21 and further comprising a plurality of beads dispersed insaid layer of liquid crystal material.
 26. A cell wall for a bistablenematic liquid crystal device, said cell wall having a first surfacewith a patterned surface profile to provide two different pretilt anglesin the same azimuthal plane to molecules of liquid crystal material,wherein said patterned surface profile comprises at least one pillar.27. A device including a cell wall according to claim 26 wherein theheight of each of said plurality of pillars is within the range of 1-3μm.
 28. A device including a cell wall according to claim 26 wherein thewidth of each of said plurality of pillars is within the range of 5-50μm.
 29. A device including a cell wall according to claim 26 wherein thewidth of each of said plurality of pillars is greater than 50 μm.
 30. Adevice including a cell wall according to claim 26 wherein said pillarsare embossed.
 31. A liquid crystal device providing two stable andoptically distinguishable liquid crystal configurations, said devicecomprising a cell wherein said cell has a cell wall according to claim26.
 32. A liquid crystal device providing a first stable liquid crystalconfiguration and a second stable liquid crystal configuration, saidfirst stable liquid crystal configuration being opticallydistinguishable from said second stable liquid crystal configuration,said device comprising a cell, said cell having at least one cell wallhaving a first surface to provide two different pretilt angles in thesame azimuthal plane to molecules of liquid crystal material, whereinsaid first stable liquid crystal configuration is a twisted molecularconfiguration.
 33. A device according to claim 32 wherein said secondstable liquid crystal configuration is a non twisted molecularconfiguration.
 34. A device according to claim 32 wherein said moleculesof liquid crystal material exhibit positive dielectric anisotropy.
 35. Adevice according to claim 32 wherein said molecules of liquid crystalmaterial exhibit negative dielectric anisotropy.
 36. A bistable nematicliquid crystal device comprising; a first cell wall and a second cellwall, said first cell wall and said second cell wall enclosing a layerof liquid crystal material, wherein said first cell wall has a firstsurface treated to provide two different pretilt angles to molecules ofliquid crystal material and said second cell wall has a first surfacetreated to provide monostable alignment to molecules of liquid crystalmaterial, wherein said bistable nematic liquid crystal device providestwo stable and optically distinguishable liquid crystal configurations.